1. Field of the Invention
The embodiments of the present invention refer to a reversible system for injecting and extracting gas for a fluid rotating machine, in particular for a centrifugal compressor.
2. Description of Related Art
As known, a compressor is a machine capable of raising the pressure of a compressible fluid (gas) by using mechanical energy. Among the various types of compressors used in the industrial field process systems there are the so-called centrifugal compressors, wherein the energy to the gas is provided in form of centrifugal acceleration due to the rotation, generally driven by a driver (electric motor, vapor turbine or gas turbine), of a member referred to as rotor made up of one or more wheels or centrifugal rotors.
Centrifugal compressors may be provided with only one rotor, in the so-called single stage configuration, or several rotors arranged in series, thus referred to as multistage compressors. More precisely, each of the stages of a centrifugal compressor is usually made up of a pipe for suctioning the gas to be compressed, by a rotor, which is capable of providing kinetic energy to the gas, and a ducting for connecting a rotor to the following one, whose task is that of converting the kinetic energy of the gas discharging from the rotor into pressure energy. In particular, these ducts are made up of a first pipe portion for discharging from a rotor, referred to as a diffuser, a substantially U-shaped fitting referred to as “cross-over”, and a second pipe portion for introduction into the subsequent rotor, referred to as return channel.
Modern multistage centrifugal compressors used in the petrochemical industry may be designed with systems for injecting and/or extracting gas on intermediate stages, also referred to as side streams. Some typical applications of these compressors are represented by machines used in refrigerators cycles, which use high molecular weight gases, such as propane and propylene, which are injected or extracted on intermediate stages depending on the process requirements. Extraction or injection of the gas is usually performed by means of worm screws or volutes made in the stator parts of the compressor, between two consecutive stages, in connection with an external flange.
Generally, the worm screw is substantially shaped to form a “spiral”, which is extended circumferentially around the axis of the machine and which has a section suitably shaped to reduce the fluid dynamic loss to the maximum.
Given that efficiency and, generally, proper operation of a compressor depend on the aerodynamic losses in the stator parts, the injection and extraction worm screws must be designed to optimize in the geometry thereof to allow the correct flow of the gas both from inside the compressor to an external flange, for extraction systems, and from an external flange into the compressor, for the injection systems.
Up to date, centrifugal compressors provided with worm screws and respective systems for injecting and extracting gas on intermediate stages do not allow optimizing the gas stream, both when injecting and extracting, when such systems are installed on a single multistage compressor case. This is mainly due to the fact that traditional systems for injecting and extracting gas on intermediate stages provide for the use of a worm screw for each stage, leading to high loss of head when the gas is made to flow into the components of the system in the direction opposite to that provided for according to the design. In other words, the high velocities of the gas inside a compressor are such to create high loss of head should an extraction worm screw be used for injecting gas and vice versa.
An optimized operation in both modes is thus possible only if the centrifugal compressor is provided with a plurality of distinct cases operatively connected to each other by means of pipes that connect the outlet flange of a compressor case to the suction flange of the subsequent case. In other words, when a reversible system for injecting and extracting gas on intermediate stages with good efficiency is required, it is necessary to interrupt the compression with machines separated, executing and connecting the side stream outside the machine directly on the process pipe. This however implies increasing costs (for manufacturing the machine, laying the foundations, etc.) and lower reliability (higher number of auxiliary devices, connection pipes, etc.).